Just FYI, this came up today on my first PCB upload test to SparkFun's BatchPCB service. I used the default text, set to "always vector fonts", and wrote text in the bottom copper layer. SparkFun's own tutorials suggest doing this, since silkscreening on the bottom is rare and an extra fee.

At the default EAGLE text settings, these fonts were rejected by the BatchPCB "checker bot" because the "traces" were not thick enough. The bot sends back a color-coded image: green is good, purple is too close to other traces, orange is too thin to be sure of a solid trace.

Whups. It would be fine, except the bot won't let you buy a "failed" board.

So if you're in a position like this, you have to use the "i" button, choose the offending text-on-copper object, and adjust the "Ratio" from its default 8% to something bigger like 15%. This makes the font strokes thicker, the font heavier and bolder.